It is well known that the scattering of incident radiation such as x-rays, gamma rays, cathode rays, etc. from a sample of material can yield information about the atomic structure of the material. When such a beam of radiation strikes a sample, a diffraction pattern of radiation strikes is created, which has a spatial intensity distribution that depends on the wavelength of the incident radiation and the atomic structure of the material. The following discussion will focus on x-ray powder diffraction as a nonlimiting example.
If the sample is an oriented single crystal, the diffraction pattern consists of a series of spots, corresponding to a projection of the reciprocal lattice of the crystal. If the molecules of the sample are randomly oriented (e.g. polycrystalline, amorphous, or powdered) the diffraction pattern becomes a series of cones, concentric with the incident beam, with the intensity and angle of the cone revealing information about the material structure. The study of diffraction patterns from powdered materials is generally referred to as powder diffraction. The function of a powder diffraction detector is to determine the angle and the intensity of this diffraction pattern.
Photographic film is one widely used detector for powder diffraction. When the film is exposed to x-rays, a latent image of the diffraction pattern is formed and when the film is developed, the density of the developed image is a measurement of the x-ray intensity in the diffraction pattern. If a flat sheet of film is placed perpendicular to the x-ray beam, the powder diffraction pattern image is a series of concentric circles. It is often desirable to record a wide diffraction angle, approaching 90 degrees. To accomplish this, in a so called Debeye-Scherrer camera, a strip of film is positioned on a cylindrical surface that intersects the beam, with the axis of the cylinder passing through the sample. The film strip records a portion of the diffraction pattern comprising arcs of the diffraction circles. While film has very good spatial resolution and can record large area patterns, it suffers from several drawbacks. Because film absorbs only a small percentage of the x-ray quanta incident on it and because it has a relatively high background noise in the form of chemical fog, film is a slow or insensitive detector of x-rays. I.e., high doses of x-rays must be given the sample to achieve a readable image with acceptable signal to noise ratio. While conventional x-ray intensifying screens can be employed to increase the sensitivity of film based powder diffraction systems, it is difficult to maintain intensity calibration with such screens. Film has a limited range of density linearity versus exposure, typically less than two orders of magnitude, so that widely differing intensities cannot be measured on the same piece of film. Also, film must be processed with wet chemistry which is an inconvenience. Finally, to utilize computers to analyze the data, the film must be scanned with a densitometer to convert densities to digital data, a time consuming intermediate step. Generally the scanning is performed on the film strip in a direction perpendicular to the arc segments of the diffraction pattern produced by the Debeye-Scherrer camera.
Various electronic detectors have been used to measure powder diffraction patterns, such as charge coupled devices, wire proportional counters, scintillators and the like. Such detectors efficiently absorb x-ray quanta and have little noise, so they are more sensitive than film, and produce digital electronic data directly. However, electronic detectors usually have a maximum counting rate so they cannot record strong intensities without dead time losses. Also, they have limited size so they can cover only a small area at one time. To form a complete scan the electronic detector must be moved until it has sequentially covered the entire area, resulting in additional exposure time. Recently, position sensitive detectors, which measure the position of quanta along a line instead of at a point, have been employed. Such position sensitive detectors must also be moved to cover an area.
The accumulation of accurate data for powder diffraction can take many hours or even days with conventional apparatus, this is a severe limitation on throughput when it is desirable to rapidly examine many samples.
Another technology for recording x-ray intensities is based on stimulable storage phosphors. Such storage phosphors when exposed to high energy radiation such as x-rays, cathode rays, etc., store a portion of the incident radiation. If the exposed phosphor is then exposed to stimulating radiation, such as visible light or heat, the phosphor will emit radiation in proportion to the stored energy of the original exposing radiation. Screens formed from such stimulable phosphors have been discussed in the literature (J. Miyahara et al., Nuclear Instruments and Methods in Modern Physics Research A246(1986), (572-578) as having very desirable properties, in terms of sensitivity and exposure latitude, for the detection of x-ray diffraction patterns from single crystal samples. Because stimulable phosphors efficiently absorb incident quanta and have very low background, they are 5-50 times more sensitive than photographic film. Stimulable phosphor x-ray imaging systems have resolutions on the order of 0.1 mm and can be made in large area formats, with millions of effective image sensing elements over a large area simultaneously integrating intensities, with no counting rate limitations. The stimulated signal is linearly related to the radiation exposure over at least 5 orders of magnitude. However, x-ray diffraction apparatus employing stimulable phosphors would improve the amount of exposure required only by the ratio of phosphor screen to film sensitivity. An even larger improvement is desirable.